1. Field of the Invention
The invention relates to a method and an apparatus for roughening a support for radiation-sensitive coatings. The surface of the support may be roughened electrochemically or may be roughened mechanically followed by electrochemical roughening. The electrochemical roughening is performed in an aqueous electrolyte bath by applying three-phase or single-phase current to electrodes which face the support while the support is passed through the electrolyte bath.
2. Description of the Related Art
Roughened supports may be used to produce presensitized printing plates. The supports may be formed of materials which are typically processed in plate or strip form, such as a metal. Preferably the supports are formed of aluminum. The roughening of, for example, the aluminum strips to produce printing plates, for example, is typically carried out mechanically, chemically or electrochemically, or by a combination of these roughening methods. During mechanical roughening, the surface structures typically have pyramid-like shapes and are oriented differently in the longitudinal and transverse directions (anisotropy), while electrochemically roughened surfaces generally have a sponge-like structure with a large number of wells and depressions and a uniform geometry in the longitudinal and transverse directions (isotropy).
Mechanical roughening has the advantage over purely electrochemical roughening of using less specific energy per square meter of the surface of the support. However, mechanical roughening suffers from the disadvantage of an excessively coarse surface on which crystalline structures are present, in addition to the pyramid-like structures. Owing to the anisotropy of the mechanical roughening process, it is significant to the printing process whether the printing plate has been cut along or across the strip.
Mechanical roughening methods are in general granulation methods, such as wire or brush granulation, or emery grinding. Electrochemical roughening on the other hand is in general carried out by electrolytic etching in an aqueous electrolyte solution.
German Patent No. 1,962,728 describes a method for the continuous production of a lithographic surface on a metal strip by wet grinding and electrochemical treatment in an electrolyte. The electrolyte is used to wet the metal surface during grinding and the electrochemical treatment is carried out immediately following the grinding process. For this purpose, a fine-granular grinding agent is suspended in the electrolyte and the grinding-agent suspension is blasted onto the moving strip using a broad jet extending over the entire width of the metal strip. The electrolyte is, for example, an aqueous acidic or aqueous alkaline bath.
In the granulation method described in German Patent No 2,130,391, the aluminum plate is first roughened by grinding with a moist emery compound. After rinsing and possibly cleaning the plate, the granulated surface of the aluminum plate is anodized in sulfuric acid using direct current at a voltage in the range from about 10 to 20 V with a current density in the range from about 1 to 2.2 A/dm.sup.2 of granulated surface. Finally, the granulated and anodised surface of the aluminum plate is treated with a priming substance in order to improve the bond between the support material and the light-sensitive coating to be applied to the surface.
German Patent No 2,650,762-B discloses a method for electrolytic granulation of aluminum substrates for lithography using a single-phase current in an electrolyte containing hydrochloric acid or nitric acid. A single-phase voltage is applied, whose anode voltage is greater than the cathode voltage and the ratio of the cathodic charge input to the anodic charge input is less than unity. The anodic half-period of the single-phase current is set to be equal to or shorter than the cathodic half-period. The diameter and the depth of the pores and holes in the surface of the aluminum substrate may be varied, as required, by selecting a suitable ratio between the cathodic and anodic charge input, governed by the voltage setting. The frequency of the regulated single-phase current is not limited to the typical single-phase frequency range, i.e., 50 to 60 Hz. However, higher frequencies result in finer pores on the granulated surface.
German Patent Specification No. 3,012,135 describes a method for producing a support for lithographic printing plates in which the surface of an aluminum plate is mechanically roughened by wet grinding. Aluminum is chemically etched away from the surface of the plate and an electrical current with a waveform which is obtained by alternately changing the polarity, is subsequently applied to the plate in an acidic aqueous solution. The anode and cathode voltages are 1 to 50 V. The ratio of the amount of charge formed with the plate as the anode to the amount of charge formed with the plate as the cathode is 0.5/1 to 1.0/1. The electrolysis is carried out such that the current density when the plate is the anode is not less than 20 A/dm.sup.2 and the amount of charge formed with the plate as the anode is 200 Coulomb/dm.sup.2 or less. The plate is then subjected to anodic surface oxidation.
European Patent No. 390 033 B1 describes a method for roughening a support in which the frequency of the three-phase or single-phase current is chosen to be greater than or equal to 50 Hz to 300 Hz and in which the frequency is normally set to be higher as the speed at which the support is transported through the electrolyte bath increases. For this purpose, the electrodes are connected in the electrolyte bath to the secondary of a first three-phase transformer, whose primary is connected to a three-phase power transformer via a three-phase frequency converter and control transformers. The three-phase frequency converter converts the mains frequency of this three-phase current in a range which is greater than or equal to 50 to 300 Hz, at a voltage of between 1 and 380 V, for the individual phases of the three-phase current.
German Patent No. 3,910,450 C2 describes a method for producing a printing plate support in which the printing plate support surface is electrochemically roughened in an acidic electrolyte using single-phase current at a frequency of 80 to 120 Hz and in which the ratio of the anode time to the period is 0.25 to 0.20. The method requires a high level of complex circuitry due to the large amount of electrical power used which results in problems in the distribution of the current to the individual electrodes since it is difficult to design a connection of a pole to the printing plate support.
German Patent No. 3,842,454 C2 relates to a method for electrolytic surface roughening of an aluminum plate using a single-phase current. According to the disclosure, an electrically insulating organic or inorganic initial coating is formed on the aluminum plate before the electrolytic surface roughening. This method not only involves additional complexity to form this coating but also requires appropriate technical precautions due to the influence of non-sinusoidal single-phase currents used in the method. Even a very uniform initial coating does not prevent the formation of transverse strips, since the transverse strips are essentially caused by non-inear electrical characteristics of the surface of the printing plate support at the start of the roughening process.
German Patent No. 3,910,213 A1 describes roughening a support using a single-phase current at a higher, variable frequency which leads to a reduction in the intensity of the transverse strips. However, the method involves increased complexity of electrical facilities and limits the frequency range of the single-phase current which can be used for optimum structuring of the surface of the printing plate support.
Roughening of the printing plate support at specific transport speeds, such as those proposed in European Patent No. 585 586 B1, may result in constant action on each part of the printing plate support during the positive and negative half-cycles of the single-phase current. However, this ignores the fact that the transverse strips are essentially formed by the half-cycles present upon entering the zone of the single-phase roughening.
The known methods and devices attempt to account for or reduce the formation of the so-called transverse strips throughout the entire time when the printing plate support is passing through the single-phase roughening zone; however, prior methods do not prevent transverse strips from being formed at the point where the printing plate support enters the area in which the single-phase or three-phase electrodes operate.
The aim of combining mechanical and electrochemical roughening is to link the advantages of both methods together. It is expected that the roughened surface of the metal support will have finely superimposed wells and depressions, which are produced by the electrochemical roughening. It has unfortunately been found, however, that in addition to the pyramid-like structures from the mechanical roughening, relatively large holes occur as a result of the electrochemical roughening. A problem with electrochemical roughening using single-phase current or with the superimposition of electrochemical roughening by means of single-phase current on a mechanically roughened surface of a metal support at very high metal support processing speeds, is that the so-called electrical surface discharges occur in time with the single-phase voltage. These surface discharges are visible in the form of strips on the surface of the metal support. These disturbing surface discharges are caused by the continuous change in polarity of the single-phase voltage applied to the electrodes.
The surface discharges in the form of strips, also referred to as transverse strips or current strips, detract from the visual impression of the product. If the strips are particularly severely pronounced, the quality of the product may be affected as well. The formation of these transverse or current strips generally increases as the transport speed of the printing plate support increases. These strips are caused at the start of the roughening process by single-phase or three-phase current. The electrical behavior of the printing plate support and of the electrolyte at the start of the roughening process is non-linear, and varies as the roughening progresses. When the printing plate support enters the area in which the three-phase or single-phase electrodes separate, this non-linear behavior leads to the formation of the transverse strips, depending on whether the single-phase voltage was in a positive or negative half-cycle when the support entered the roughening zone.